It is generally known that acoustic measurement can be used to determine properties of a material, such as, a wood product. These properties may include, for example, stiffness, strength, elasticity and other characteristics. In some embodiments, in which properties of a wood product are being ascertained, a stress wave is induced into the wood product. Next, a measurement is taken with respect to the time in which the stress wave travels from a first end to a second end of the wood product. From this time interval, a velocity of the stress wave can be determined via the equation:v=d/t
Where “v” is velocity of the stress wave; “d” is the distance traveled by the stress wave; and “t” is the time period of travel. This method of determining velocity is commonly referred to as a “time-of-flight” method. The velocity can, for example, be correlated to a modulus of elasticity for the wood product, which is an indicator of the stiffness of the wood product.
Another method for determining properties of a material is through the use of resonance frequency. In this method, the material may be contacted, or struck, to induce a wave within the material. The different frequencies, or harmonics, at which the material resonates based on the induced wave may be measured. Higher order harmonics may be used to discern a fundamental frequency for resonation. Using the obtained fundamental frequency, the velocity of the wave can be determined via the equation:v=2fL
where “v” is velocity of the stress wave; “f” is the fundamental frequency; and “L” is the length of the material.
This velocity may also provide information as to the stiffness or other characteristics of the material. Use of resonance frequency is well known when discerning properties of, for example, wood products.
However, many properties of materials, such as, for example, wood products, are not homogeneous throughout the product. Time-of-flight measurement tends to estimate the properties of the high stiffness path within a wood product; whereas resonance frequency measurement tends to estimate the average properties within that product. However, many wood products have imperfections such as knots or other structural defects. These defects can significantly alter the data measured via time of flight measurements and/or resonance frequency measurements as well as the overall findings regarding wood product properties.
In an example, FIG. 1 shows typical waveforms obtained from start and stop sensors used to measure acoustic velocity in wood by the time-of-flight technique. With this technique, it can be difficult to determine the exact time of arrival of the acoustic energy at the downstream (stop) sensor. The precise arrival time should be the instant when the acoustic energy begins to appear at the detector, thereby corresponding to the point when the detector output begins to rise. Unfortunately, most methods of detecting this leading edge are very sensitive to noise. Another technique commonly used to establish time of arrival is to locate the point where peak amplitude is reached. In the example shown in FIG. 1, this time-of-arrival ambiguity results in 20% uncertainty in the estimate of acoustic velocity. This data was taken on an 8 ft pine 2×4 using a FAKOPP® device, with start and stop sensors placed 7.5 feet apart.
FIG. 2 illustrates a typical output from an accelerometer attached to the same piece of lumber. The display shows the waveforms that are resonating within the lumber 2-5 milliseconds after a stress wave is induced. The data was taken from the same test used to generate FIG. 1. FIG. 3 is a display of the Fourier transform of the data shown in FIG. 2. This transform plot shows that the lumber piece is resonating at several frequencies. In this case, there is significant energy at 610 Hz, 1099 Hz, 1343 Hz, 2076 Hz, and 3541 Hz. These frequencies correspond to acoustic velocity estimates (ft/sec) of 9760, 16485, 21488, 33216, and 56656 respectively. It can be difficult to determine which of these velocity estimates represents the compression wave of interest. In this example, both the 9760 ft/sec and the 16,485 ft/sec estimates fall within the range of legitimate stress wave velocities normally associates with dry lumber.
A need, therefore, exists for a method for using both time of flight and resonance frequency measurements to provide greater accuracy and/or precision when determining a velocity of a wave within a material as well as to determine properties of the material.